1. Field of the Invention
The present invention relates to a semiconductor integrated circuit for the sensorless driving of a brushless motor, and a sensorless driving system.
2. Background Art
Conventionally, in the sensorless driving of brushless motors, induced voltages are detected which are obtained from exciting coils by the rotations of rotors. When the detected induced voltage is shifted in phase by, e.g., 30 degrees from a zero cross point crossing the midpoint voltage of the rotor, a commutating operation is performed.
Regarding this sensorless driving method, a method of detecting the rotational position of a rotor is available. The rotational position is detected by outputting, e.g., a sensing pulse when the position of the rotor is detected during a short brake in which exciting coils are connected to the ground.
Because of this sensing pulse, a complicated circuit is necessary for detecting the position of the rotor and it is further necessary to consider a resistance between the terminals of a brushless motor and an inductance between the terminals of the brushless motor.
As prior art, a commutation abnormality detecting apparatus is available which includes: polarity identifying means fed with a three-phase induced voltage outputted from a three-phase brushless motor to identify the polarity of the voltage, actual commutation mode generating means for estimating a commutation mode based on the identification result, actual commutation mode generating means for generating a commutation mode determined in a predetermined order with reference to the reference voltage of the induced voltage, and commutation mode comparing means for deciding whether the commutation mode estimated by the commutation mode estimating means and the commutation mode generated by the actual commutation mode generating means match with each other (e.g., see Japanese Patent Laid-Open Publication No. 2000-50679).
In the prior art, it is detected whether the commutation is normal or not during the commutating operation but the rotational position of a rotor is not detected during a short brake.